The present invention relates generally to noise reduction, and in particular to a noise reduction circuit which eliminates impulse noise that occurs in audio signals using a linear interpolation technique immune to white noise. The invention is particularly useful for applications where the audio signal level is relatively low in comparison with the white noise level.
One method currently available for suppressing impulse noise that contaminates audio signals involves reducing the transmission gain or shutting off the transmission path of the signal as long as the noise is present. Another method involves detecting the amplitude of the wanted signal on the rising edge of an impulse noise and retaining the detected amplitude in the presence of the impulse noise. While these methods are effective in suppressing impulse noise, the noise-affected portion of the signal is not reconstructed, resulting in unnatural sound. To overcome this problem modern digital audio systems utilize linear interpolation technique to predict the original waveform of the noise-affected portion by linear interpolation. This type of systems requires complicated, expensive circuitry, not suitable for moderate cost equipments.
The aforesaid copending U.S. application discloses an impulse noise reduction circuit for an audio signal whose high frequency components are preemphasized. The signal is passed through a first deemphasis circuit which partially deemphasizes the signal and through a first sample-and-hold circuit which tracks the waveform of the signal when no impulse noise is present and holds the signal level in response to the impulse noise, while at the same time cutting off the transmission path of the noise reduction circuit. The output of the first sample-and-hold circuit is applied on the one hand to a second deemphasis circuit thence to the output terminal and on the other hand to a feedback circuit which includes a differentiator. The deemphasis circuit coacts with the first deemphasis circuit to provide a deemphasis characteristic complementary to the preemphasis characteristic. The slope ratio of the audio signal is detected by the differentiator and sampled by a second sample-and-hold circuit in response to that impulse noise as an indication of the position of the noise of interest in the audio signal waveform. The sampled signal drives a voltage-controlled bidirectional constant current source to linearly vary the voltage sampled by the first sample-and-hold by performing linear charging and discharging of the capacitor thereof.
Since the input signal to the differentiator is not arranged to pass through the second deemphasis circuit, the white noise is present in that input signal and detected by the differentiator so that it accenturates the white noise, resulting in an error in the interpolation voltage.